We do a lot more than just talk Gretsch.

What in the world did I do to my Club?

Well, I finally sat down with my Spruce Club and raised the pickups with some pickup spacers I'd gotten from Setzer a while back. Output had been about 60% of my 6120. Fine.

Well, once I got everything back in place, and before plugging it in, I thumbed a few notes up the fretboard. My ears were greeted to some shrill, dissonant overtones and harmonics...pinging and poinging...

If it's something that's happening acoustically, check if things are vibrating.
Put your finger on the various components with a bit of pressure, pickups, pickguard switches etc and then strum or pluck a note. See if you can make it go away. Sometimes the strings behind the bridge will make nasty overtones. Another thing to check.

And if your Dearmond pole pieces are too close to your strings they can pull them out of tune/rob them of sustain. It sounds mechanical though and likely easily remedied

Nah, it's a case of, 30 minutes ago it was terrific -- before I laid hands on it. Then it's not. If I get unlazy I might do an audio clip. But if I got that unlazy, I'd probably spend enough time fiddling with it to figure out what I did. To me, it sounds like a bridge-to-tailpiece thing...like the tailpiece is pinging. I checked that strings were fully seated......

I can't get rid of the Melita on this. I don't dislike it and it's purdy. I've got a TA on the 6120 where it must be.

On Car Talk, they sometimes talk about using a mechanic’s stethoscope, or a rubber tube.

They sell stethoscopes, medical and mechanic’s, down at the local surplus store. They have been on my list of things to buy for a couple of years because I wanted to try one like on that Traveler guitar.

You could eliminate the 'wolf tone' issue between the bridge and tailpiece easily by threading something between the strings in that area to dampen them.

It's possible that by changing the pickup height, you brought the pickup wiring into contact with the structure inside the guitar and it's vibrating as the guitar resonates. Harder to diagnose, but a glance inside with an inspection mirror might allow you to spot any potential trouble spots.